Is DPreview right about 40D DR at ISO1600? 8.9EV

[Panopeeper]

I have plotted the complete RGB sensor curve response of my 350D. The plot is made of 37 shots, 6 f-stops both sides of the camera metering point

Guillermo,

this is not a reliable way just for what you are aiming at; you have to achieve the goal with a single shot. Shooting a Stouffer wedge is the way to go (the following crop is before WB); the wedge is much longer, going from black to almost fully transparent.

http://www.cryptobola.com/PhotoBola/image/StoufferNoWB.gif

http://www.cryptobola.com/PhotoBola/image/...ramStouffer.gif

The linearity is fine except for one point in the highlights of the B channel, which seems to provide a slightly higher than expected value once the R and G channels totally burnt

This *may* be due to slight irregularity of the shutter time.

Anyway, have you verified the raw image, if there is perhaps a slight clipping in the blue? It is possible, that the saturation levels are different for the three channels.

[John Sheehy]

Qualitatively, the knee in each curve represents the point above which noise is limited by photon statistics, and below which noise becomes more and more dominated by sensor read noise.

If you project the shot-only part of the curve as a straight line without read noise, you can clearly see that read noise robs ISO 100 of a few stops of DR.  The visual experience would be even greater than that difference, IMO, as shot noise has a much more aesthetically pleasing look than read noise, and looks like a marbled texture in deep shadows areas, with blacks truly approaching black, whereas with read noise, contrast in the deep shadows is obliterated by the blanket of read noise that covers both black and near-black alike.

So, if your criterion is the range of EV over which SNR>1, that's the range of EV along the horizontal axis for which the curve is positive.  For instance, 11.1 stops for ISO100 (from 2.5 to 13.6), on down to 9.2 stops for ISO 1600 -- roughly comparable to the engineering definition of DR.  If you want a more stringent criterion, say SNR > 4=2^2, then that's the range above 2 on the vertical axis; this gives a DR of about 9 stops at ISO 100, etc. Direct sensor measurements are not "useless" as the first response in this thread seemed to imply; they contain all the data needed to reconstruct the camera's noise profile and signal-to-noise ratio at any exposure level.
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Just stating a single standard deviation, however, noise is not always represented properly in terms of its visibility, even with a fixed or standardized pixel viewing magnification.  1-dimensional noises are more visible than their statistical strength suggests, and low-frequency noises are, as well.  Neither reduces as much as high-frequency 2-dimensional noise, either, with downsampling, binning, or decreased image display magnification.

To be thorough, these things should be taken into account as well, however, current technology is moving away from 1-dimensional and low-frequency noises, it seems.

[Guillermo Luijk]

This *may* be due to slight irregularity of the shutter time.

Anyway, have you verified the raw image, if there is perhaps a slight clipping in the blue? It is possible, that the saturation levels are different for the three channels.

Yes I did, and there was no clipping. However any clipping should lead to a lower than expected level, but in this case it was higher!

Anyway, have you verified the raw image, if there is perhaps a slight clipping in the blue? It is possible, that the saturation levels are different for the three channels.

I understand, but I don't have this step wedge.

Anyway do you think a printed step wedge provides more precission than camera's shutter speed? I not only mean the quality of reflectance in the printed stuff but also to get a very uniform lighting is difficult. I have analysed shots over a gray card, and even when our eyes see that luminosity is uniform on its surface there are stong differences, of more than 1 f-stop (see here), so if the step wedge extends over a considerable distance we could have problems in the precision of our measures for light fall offs on the card.

Regards.

[Panopeeper]

Anyway do you think a printed step wedge provides more precission than camera's shutter speed? I not only mean the quality of reflectance in the printed stuff but also to get a very uniform lighting is difficult

I'm not sure; you have two variables, the printing and the lighting. Though such a strip does not need to be large.

The Stouffer wedge has to be back-lit, with the surrounding covered. If I were as interested in what you are doing as you appear, I would purchase one, the T4110, it costs US$30, see http://www.stouffer.net/TransPage.htm See the Specifications section as well, for the tolerances. I have no idea, if and where else it can be purchased.